It is known that polymeric films often have poor handling properties which may result in difficulties in winding the films into high quality reels and inefficient passage through processing, for example, slitting, equipment. Film handling properties can be improved by increasing the surface roughness of the film, suitably by the use of coatings, or alternatively by incorporating fillers, ie organic or inorganic particles into the film. A combination of coatings and fillers may be used to improve film handling properties. The problem with using coatings to improve film handleability is that they limit the range of uses to which the film may be applied because of the difficulty in applying additional coating layers which may be required, for example, to provide antistatic, adhesion promoting or release properties. Filler incorporated into a coating layer is susceptible to abrasion and loss from the coating layer.
A wide range of fillers have been incorporated into films to improve handling properties, such as titanium dioxide, calcium carbonate, glass, barium sulphate, silica, kaolin, china clay, zeeospheres and calcium phosphates. However, the presence of these fillers in the film results in a depreciation in the optical clarity and an increase in the haze, of the film.
Optical clarity and transparency are important criteria in a wide range of film applications, such as packaging, metallised films, reprographic films and films for general industrial use. There is a continuing need for films exhibiting high light transmittance, low haze and excellent handling properties. One way of reducing this problem is to produce a clear base film with a thin layer of material containing a filler, which can act as an anti-blocking agent, without reducing the overall transparency of the composite film to unacceptable levels. U.S. Pat. No. 4,533,509 describes polyester films of this type.
Polyester film composites comprising a layer of transparent homo-polyester and a layer of transparent copolyester are described in GB Patent No 1,465,973. European Patent No 35835 describes a similar polyester film composite wherein the filler in the copolyester layer has an average particle size greater than the thickness of the layer. The filler particles protrude through the copolyester layer yielding a film with good anti-block properties whilst increasing the transparency and heat-sealability thereof. However, the aforementioned properties are only achieved at certain copolyester layer thickness/filler particle size ratios, such that any variation required in the copolyester layer thickness (for example for a different commercial application) necessitates a change in the filler particle size. This situation can result in a range of different fillers being required for different applications.
European Patent No 229670 discloses a polyester monofilm comprising 0.005% to 1% of silicone resin particles having an average particle diameter of 0.01 .mu.m to 4 .mu.m. The film is used in magnetic recording media.
We have surprisingly overcome or substantially reduced one or more of the aforementioned problems.